Am I correct in thinking that if I add a coax cable to move the lnb a bit farther from the receiver it must be 75 ohms?
RG-6 is easy to find, get some F connectors too or have the TV or hardware shop make you what you need.
Yes it must be 75 ohm - - the impedance of the LNB. Ken
I cheated plenty using 75Ω coax stripped from old network installs for feedline between dipoles and 50Ω radios it worked fine even for QRP and HT to amateur satellite where line and antenna loss is the big thing you can fix(btw dipole is ~70Ω), not as good as a match but for a short run it shouldn’t matter in a noticeable way especially between a LNB and receiver. In my experience you need to go to a amateur or commercial radio shop to find 50Ω stuff like rg58.
Just use a cable and connector well suited for ~1GHz and you will do fine, in any case 75Ω RG-6 for satellite dish installs is what you can buy in any store.
(edit)
But that doesn’t address the impedance mismatch which can cause some of the signal to reflect back to the teeny tiny finals in the LNB which I doubt would be affected at all by years of absorbing it’s entire output as heat.
Here is a datasheet of common cables
So RG-6 is loss nearly ideal for the freq (900mhz-1.1ghz) we are using while rg-58 gets pretty lossy at 56dB/100m of cable but that is still only 0.5dB/M or 0.13dB/Ft. I even bother adding this all as someone searching the internet for the answer has the data available at hand rather than having to re-ask later.